Designing a Roof Garden - Plant Selection

Designing a Roof Garden - A Designers Guide

No group of plants can be said to be unsuitable for use in a roof garden. Large forest trees may be impracticable because of problems with anchorage, but other than that almost any plant that is used in general landscape will prosper. However, before selecting plants it is important to consider how they will fair in the environment which will exist on the roof. These factors include:

The danger of exposure is often exaggerated. Plants on roof gardens rarely show evidence of wind pruning. However, it is prudent to avoid herbaceous plants or those which produce fragile flowers if the garden is exposed.

The need to tolerate drought needs to be balanced with the presence or absence of an irrigation system, and the volume of substrate. If the desire is to grow a typical range of decorative plants the need for an irrigation system cannot be over emphasized. It is not always possible to irrigate all gardens, and in recent years much has been learnt about the ability of herbaceous plants to resist prolonged periods of drought.

For example, areas of grassland that are apparently dead at the end of summer do recover in a very short time. Only herbaceous plants with tough perenating organs or plants that will grow in arid areas survive such treatment. It is possible to produce a fairly basic drought-resistant landscape which will survive without irrigation, but it would be somewhat unsightly during periods of moisture stress and thus hardly compatible with present concepts of urban landscape design.

Unfortunately, most plants, and particularly those trees and shrubs which are commonly used in urban landscapes, cannot survive prolonged periods of drought. Among those subjects which are better able to tolerate drought, the following are a few tough fast growing favourites:- Junipers spp., Cotoneaster spp., Pinus mugo, Hedera spp.

If there is no irrigation system then the plants must possess durable penetrating organs or the ability to withstand total desiccation e.g. sedums and coarse grass. One of the most commonly used elements in any landscape is grass. However, contrary to common belief, grass has a high maintenance requirement and the finer lawn grasses are subject to many problems including fungal attack, drought damage, and intolerance to low light levels. Even a relatively brief period of drought will kill the finer grasses in a lawn.

Stability

The selection of plants for roof gardens is surrounded by myths. Some plants have been condemned because they have tap roots'. It is true that some plants do have tap roots when they are small but it is doubtful whether they have any effect after the first year. It should also be remembered that the modern nursery practice of undercutting young trees encourages the formation of a fibrous root system.

Studies have revealed that most root development is horizontal, which is only to be expected since few natural topsoils are more than 300mm thick. Logic might suggest that plants would appreciate and exploit a deep soil. However, this is not the case. Roots exploit distinct layers within the available profile and rarely deviate. This reinforces the findings of ecological studies on root systems in 'natural' plant communities, which indicate that each species tries to occupy a specific zone within the soil profile.

When selecting plants it is important to try and select plants whose root systems would naturally suit the rooting environment. For example, Birch trees (Betula pendula) and Scrub Pine (Pinus nigra) have shallow root systems and are not naturally tall. A 7m tall Birch and a 3m tall multi stem scrub pine will have root balls which are less than 500mm deep. With levelling, packing and a small amount of localised mounding they will easily fit within a 600mm profile.

Planting Density

During the 1980s research work was carried out at the University of Bath on the optimum planting density for a number of ground cover plants. One unexpected result of these studies was that during periods of drought, the plants which were closest together died long before the plants that were well spread out. This was due to competition between the plants. Although the foliage might have looked uniform the leaf area index, i.e. leaf density, of the more closely planted shrubs, was three times that of the least.

The point is that high planting densities are not good. Unfortunately many people believe that an instant effect is only achieved by increasing density. As a result it is common to see planting densities of 3 plants per square meter. Most landscape shrubs will grow at least 2m across and many 2m high so there is no need to crowd them together. The best approach is to have one substantial plant per square meter and to ensure that it develops quickly to its full potential.

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